Method of forming a semiconductor structure

ABSTRACT

A method for reducing the effective thickness of a gate oxide using nitrogen implantation and anneal subsequent to dopant implantation and activation is provided. More particularly, the present invention provides a method for fabricating semiconductor devices, for example, transistors, which include a hardened gate oxide and which may be characterized by a relatively large nitrogen concentration at the polysilicon/gate oxide interface and a relatively small nitrogen concentration within the gate oxide and at the gate oxide/substrate interface. Additionally, the present invention provides a method for fabricating a semiconductor device having a metal gate strap (e.g., a metal silicide layer) disposed over the polysilicon layer thereof, which device includes a hardened gate oxide and which may be characterized by a relatively large nitrogen concentration at the silicide/polysilicon interface to substantially prevent cross-diffusion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Patent Application Ser. No.11/756,922, filed Jun. 1, 2007, scheduled to issue as U.S. Pat. No.7,968,954 on Jun. 28, 2011, which is a continuation of application Ser.No. 10/985,573, filed Nov. 10, 2004, now U.S. Pat. No. 7,259,435, issuedAug. 21, 2007, which is a divisional of application Ser. No. 10/651,314,filed Aug. 28, 2003, now U.S. Pat. No. 7,314,812, issued Jan. 1, 2008.The disclosure of each of the previously referenced documents is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to semiconductor devices andmethods for their fabrication. More particularly, the present inventionrelates to hardening of gate oxides in semiconductor devices by nitrogenimplantation and anneal subsequent to dopant implantation and activationto increase the dielectric constant thereof and, accordingly, decreasethe effective thickness of the gate oxide.

BACKGROUND

Higher performance, enhanced reliability and greater packaging densityare constant goals of the semiconductor industry. However, as componentsof integrated circuits become increasingly smaller to meet these goals,it has become more and more difficult to produce semiconductor devicescapable of reliable, long-term operation, particularly in view of theoperational stresses each component of a state of the art semiconductordevice must endure. For instance, as surface P-channel transistorsdecrease in size, the size and thickness of the gate oxides included insuch transistors must also decrease. However, as gate oxide thicknesscontinues to be compressed, the gate oxides become increasinglypermeable to dopants included in the overlying polysilicon gateelectrodes. Further, they become less resistant to hot electrondegradation and more susceptible to breakdown voltages below normaloperating voltages.

To address these problems, various processes for hardening gate oxidesand, accordingly, reducing the effective thickness thereof, have becomehighly beneficial to the fabrication of state of the art semiconductordevices. For instance, a method well known in the art for forminghardened gate oxides involves implanting nitrogen into a semiconductorsubstrate (e.g., a silicon substrate) followed by thermal oxide growthon the top surface of the substrate. During the thermal oxide growth,oxynitride is formed at the gate oxide/substrate interface. Asoxynitride has a higher dielectric constant than pure oxide, theresultant oxide effective thickness is smaller than it would be withoutthe nitrogen implantation.

Another conventional method for forming hardened gate oxides involvesimplanting nitrogen into the gate oxide after formation thereof. Themethod includes providing a semiconductor substrate, forming a gateoxide layer over the semiconductor substrate and subjecting the gateoxide layer to a nitrogen implantation treatment. The nitrogenpenetrates the top surface of the gate oxide layer but does notinitially bind therewith. As such, the nitrogen implantation is followedby an oxidative anneal which results in the formation of oxynitride inthe gate oxide layer and at the gate oxide/substrate interface. Again,due to the increased dielectric constant of the oxynitride relative topure oxide, the resultant gate oxide layer has a smaller effectivethickness than it would have without the nitrogen implantation.

In addition to having an increased dielectric constant, relative tononhardened devices, gate oxides hardened by known methods are generallyless permeable to dopants included in polysilicon electrodes, moreresistant to hot electron degradation and less susceptible to breakdownvoltages below normal operating voltages. However, known processes forhardening gate oxides also have drawbacks. For example, in order toprevent diffusion of dopants from the polysilicon electrode into andthrough the gate oxide, known hardening processes often provide a highconcentration of nitrogen at the interface of the gate oxide and theunderlying semiconductor substrate. However, as is known to those ofordinary skill in the art, excessive nitrogen at the gateoxide/substrate interface significantly degrades transistor performance.

Accordingly, in terms of device performance and reliability, it has beenfound to be advantageous to fabricate a gate oxide layer having arelatively small nitrogen concentration at the gate oxide/substrateinterface with the bulk of the nitrogen concentration being located atthe polysilicon/gate oxide interface. The relatively large nitrogenconcentration at the polysilicon/gate oxide interface effectivelyprevents diffusion of dopants from the polysilicon electrode and intoand through the gate oxide layer while the relatively small nitrogenconcentration at the gate oxide/substrate interface confers resistanceto hot electron degradation without substantially affecting deviceperformance. Further, in addition to the nitrogen concentration at thegate oxide/substrate interface, it has been found to be advantageous fora relatively small concentration of nitrogen to be located within thegate oxide to aid in increasing the dielectric constant of the gateoxide and, accordingly, in reducing the effective thickness thereof.While some known processing techniques (e.g., rapid plasma nitridation(RPN) and decoupled plasma nitridation (DPN)) provide transistorsincluding a gate oxide having a relatively large nitrogen concentrationat the polysilicon/gate oxide interface and a relatively small nitrogenconcentration within the gate oxide and at the gate oxide/substrateinterface, such techniques are often prohibitively expensive.

At least one method has been developed in an attempt to provide atransistor including a gate oxide having some of the above-statedcharacteristics. U.S. Pat. No. 6,017,808 to Wang et al. (hereinafter“the '808 patent”) describes a method for hardening a gate oxidedesigned to provide a transistor wherein a large peak of nitrogen existswithin the polysilicon and gate oxide layers at the polysilicon/gateoxide interface, while a relatively smaller nitrogen peak occurs withinthe gate oxide layer and the underlying semiconductor substrate at thegate oxide/substrate interface. To achieve this structure, the method ofthe '808 patent requires implanting nitrogen through a first polysiliconlayer and into the gate oxide layer followed by an anneal step. Afterthe implantation and annealing steps, a first, relatively large,nitrogen peak occurs entirely within the first polysilicon layer, asecond, relatively smaller, nitrogen peak occurs at the polysilicon/gateoxide interface, and a third, relatively smaller still, nitrogen peakoccurs at the gate oxide/substrate interface. However, due to itsmagnitude, the first nitrogen peak located entirely within the firstpolysilicon layer is somewhat counterproductive because it retardsactivation of subsequently implanted dopants, such as boron, within thefirst polysilicon layer. Therefore, the method of the '808 patentrequires removal of the portion of the first polysilicon layer whichincludes the first nitrogen peak without removing the portion of thefirst polysilicon layer which includes the second nitrogen peak (i.e.,the peak occurring at the polysilicon/gate oxide interface) to form asecond polysilicon layer. Once the portion of the first polysiliconlayer including the first nitrogen peak is removed to form the secondpolysilicon layer, a third, nitrogen-free polysilicon layer may beoptionally formed over the second, nitrogen-implanted, polysiliconlayer.

As will be readily appreciated, achieving the structure disclosed in the'808 patent using the methods described therein is at best difficult,particularly in light of the continually decreasing thickness ofpolysilicon electrodes included in state of the art semiconductordevices. One of the most troublesome aspects of the method described inthe '808 patent is the need to remove only the portion of thenitrogen-implanted polysilicon layer including the first nitrogen peak.The reference teaches that this task may be accomplished using known wetetch, dry etch, or chemical mechanical processing techniques. However,the polysilicon layers used for polysilicon electrodes in state of theart transistors are exceedingly thin. The polysilicon electrodes of somestate of the art devices may be as thin as seven or fewer molecularmonolayers, and known etching and polishing processes are difficult tocontrol with sufficient precision to remove only predetermined portionsof material layers of such minute thicknesses. Moreover, in thiscontext, the polysilicon layer will include varying concentrations ofnitrogen at any given depth, and as the nitrogen concentration varies,the etch rate will also vary, making precise control of the etchingprocess even more difficult. Thus, removing only the portion of thefirst polysilicon layer, including the first nitrogen peak, is extremelydifficult, and known removal processes will most likely result inremoval of too much or too little polysilicon material, resulting intransistors exhibiting impaired performance or reduced reliability.

A further problem, that of cross-diffusion, is encountered when a metalgate strap (e.g., a metal silicide layer) is disposed over thepolysilicon layer. Cross-diffusion occurs, for example, in surfaceP-channel transistors having both P-type and N-type dopants that maydiffuse across the silicide/polysilicon interface and contaminateunderlying layers. A relatively large concentration of nitrogen at thesilicide/polysilicon interface may substantially prevent dopantdiffusion across the interface. However, known processing techniques donot provide semiconductor devices having a relatively largeconcentration of nitrogen at the silicide/polysilicon interface. Inparticular, a transistor fabricated utilizing the methods described inthe '808 patent would not alleviate cross-diffusion as the portion ofthe first polysilicon layer including the first nitrogen peak is removedtherefrom. If, in later processing, a silicide layer were to be formedover the second polysilicon layer, there would be insufficient nitrogenat the silicide/polysilicon interface to effectively substantiallyprevent cross-diffusion across the interface. As an alternativeembodiment, the '808 patent describes a method wherein a third,nitrogen-free, polysilicon layer may be formed over the secondpolysilicon layer. If, in later processing, a metal gate strap were tobe formed over the third polysilicon layer, there would be a substantialabsence of nitrogen at the silicide/polysilicon interface andcross-diffusion would probably occur.

It would, therefore, be desirable to provide a method for fabricatingsemiconductor devices, for instance, transistors, which include ahardened gate oxide and which may be characterized by a relatively largenitrogen concentration at the polysilicon/gate oxide interface and arelatively small nitrogen concentration within the gate oxide and at thegate oxide/substrate interface which may be easily incorporated intocurrent fabrication processes and is not prohibitively expensive.Further, it would be desirable to provide a method for fabricatingsemiconductor devices (e.g., transistors) that include a metal gatestrap disposed over the polysilicon layer thereof and which include ahardened gate oxide, the devices characterized by a relatively largenitrogen concentration at the silicide/polysilicon interface tosubstantially prevent cross-diffusion.

BRIEF SUMMARY

The present invention, in one embodiment, includes a method forfabricating a semiconductor device, for example, a transistor, whichincludes a hardened gate oxide and which may be characterized by arelatively large nitrogen concentration at the polysilicon/gate oxideinterface and a relatively small nitrogen concentration within the gateoxide and at the gate oxide/substrate interface. The method includesproviding a semiconductor substrate having a gate oxide layer formedthereover, depositing a polysilicon layer atop the gate oxide layer,implanting dopants into the polysilicon layer, activating the implanteddopants and subjecting the resultant structure to a nitrogenimplantation treatment sufficient such that nitrogen penetrates thepolysilicon layer, the gate oxide layer and at least a portion of thesemiconductor substrate. The dosage of nitrogen and the energy at whichit is implanted may be adjusted such that the bulk of the implantednitrogen is concentrated in the polysilicon layer while a relativelysmall concentration of nitrogen penetrates below the polysilicon/gateoxide interface. In later processing, an anneal may be performed whichalters the nitrogen concentration profile such that a relatively largenitrogen concentration is exhibited in the polysilicon and oxide layersat the polysilicon/gate oxide interface and a relatively small nitrogenconcentration is exhibited within the gate oxide and at the gateoxide/substrate interface.

In a semiconductor device having a gate oxide hardened utilizing themethod of the present invention, the nitrogen at the polysilicon/gateoxide interface acts as a diffusion barrier to prevent diffusion ofdopants from the polysilicon layer into and through the gate oxidelayer. Additionally, nitrogen concentrated at the gate oxide/substrateinterface also aids in preventing dopant diffusion; however, thisconcentration is not so great as to substantially impair deviceperformance. Further, the nitrogen within the gate oxide layer reactswith the pure oxide to form oxynitride, which has an increaseddielectric constant. Accordingly, the effective thickness of the gateoxide layer is reduced.

In another embodiment, the present invention includes a method forfabricating a semiconductor device, e.g., a transistor, which includes ametal silicide layer disposed over the polysilicon layer thereof and ahardened gate oxide, the device characterized by a relatively largenitrogen concentration at the silicide/polysilicon interface, arelatively smaller nitrogen concentration at the polysilicon/gate oxideinterface and a relatively smaller still nitrogen concentration withinthe gate oxide and at the gate oxide/substrate interface. The methodincludes providing a semiconductor substrate having a gate oxide layerformed thereover, depositing a polysilicon layer atop the gate oxidelayer, implanting dopants into the polysilicon layer, activating theimplanted dopants, depositing a metal silicide layer over thepolysilicon layer and subjecting the resultant structure to a nitrogenimplantation treatment sufficient such that nitrogen penetrates themetal silicide layer, the polysilicon layer, the gate oxide layer and atleast a portion of the semiconductor substrate. The dosage of nitrogenand the energy at which it is implanted may be adjusted such that thebulk of the implanted nitrogen is concentrated at thesilicide/polysilicon interface while a relatively small concentration ofnitrogen penetrates below the polysilicon/gate oxide interface. In laterprocessing, an anneal may be performed which alters the nitrogenconcentration profile such that a relatively large nitrogenconcentration remains at the silicide/polysilicon interface, arelatively smaller concentration of nitrogen is exhibited at thepolysilicon/gate oxide interface and a relatively smaller stillconcentration of nitrogen is exhibited within the gate oxide layer andat the gate oxide/substrate interface.

In a semiconductor device having a metal gate strap and a gate oxidehardened utilizing the method of the present invention, the nitrogenconcentrated at the silicide/polysilicon interface acts to substantiallyprevent cross-diffusion of dopants across the interface and theconcentration of nitrogen at the polysilicon/gate oxide interface actsas a diffusion barrier to prevent diffusion of dopants from thepolysilicon layer into and through the gate oxide layer. Nitrogenconcentrated at the gate oxide/substrate interface also aids inpreventing dopant diffusion; however, this concentration is not so greatas to substantially impair device performance. Further, the nitrogenwithin the gate oxide layer reacts with the pure oxide to formoxynitride and, accordingly, increases the dielectric constant of thegate oxide layer and reduces the effective thickness thereof.

Other features and advantages of the present invention will becomeapparent to those of ordinary skill in the art to which the presentinvention pertains through consideration of the ensuing description, theaccompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

While the specification concludes with claims particularly pointing outand distinctly claiming that which is regarded as the present invention,the advantages of this invention may be more readily ascertained fromthe following description of the invention when read in conjunction withthe accompanying drawings in which:

FIG. 1 is a side cross-sectional view schematically illustrating anintermediate structure in the fabrication of a semiconductor devicehaving a hardened gate oxide formed in accordance with the method of thepresent invention;

FIG. 2A illustrates areas of nitrogen concentration in the intermediatestructure of FIG. 1 after nitrogen implantation;

FIG. 2B illustrates areas of nitrogen concentration in the intermediatestructure of FIG. 1 subsequent to anneal;

FIG. 3 is a side cross-sectional view schematically illustrating asecond intermediate structure in the fabrication of a semiconductordevice having a metal gate strap and a hardened gate oxide formed inaccordance with the method of the present invention;

FIG. 4A illustrates areas of nitrogen concentration in the secondintermediate structure of FIG. 3 after nitrogen implantation; and

FIG. 4B illustrates areas of nitrogen concentration in the secondintermediate structure of FIG. 3 subsequent to anneal.

DETAILED DESCRIPTION

The present invention is directed to a method for reducing the effectivethickness of a gate oxide using nitrogen implantation and annealsubsequent to dopant implantation and activation. More particularly, theinvention is directed to a method for fabricating semiconductor devices,for example, transistors, which include a hardened gate oxide and whichmay be characterized by a relatively large nitrogen concentration at thepolysilicon/gate oxide interface and a relatively small nitrogenconcentration within the gate oxide and at the gate oxide/substrateinterface. Additionally, the invention is directed to a method forfabricating a semiconductor device, e.g., a transistor, having a metalgate strap (e.g., a metal silicide layer) disposed over the polysiliconlayer thereof, which device includes a hardened gate oxide and which maybe characterized by a relatively large nitrogen concentration at thesilicide/polysilicon interface to substantially prevent cross-diffusion.The particular embodiments described herein are intended in all respectsto be illustrative rather than restrictive. Other and furtherembodiments will become apparent to those of ordinary skill in the artto which the present invention pertains without departing from itsscope.

With initial reference to FIG. 1, a cross-sectional view of anintermediate structure 10 in the formation of a semiconductor device,e.g., a transistor, fabricated according to the method of the presentinvention is illustrated. It should be understood and appreciated thatthe methods and structures described herein do not form a completeprocess for manufacturing transistors or other semiconductor devices.The remainder of the process is known to those of ordinary skill in theart and, therefore, only the process steps and structures necessary tounderstand the present invention are described herein. Additionally, itshould be understood that while the depicted method illustratesformation of a gate stack for use in a transistor, the method may alsobe applied to any structure wherein a reduced oxide electrical thicknessis desired including, without limitation, capacitors. It should befurther understood that the figures presented in conjunction with thisdescription are not meant to be actual cross-sectional views of anyparticular portion of an actual transistor or other semiconductordevice, but are merely idealized representations which are employed tomore clearly and fully depict the method of the invention than wouldotherwise be possible. Elements common between the figures maintain thesame numeric designation.

The method of the present invention includes providing a semiconductorsubstrate 12 having a gate oxide layer 14 formed thereover. Thesemiconductor substrate 12 may be formed of any suitable material knownto those of ordinary skill in the art, and the gate oxide layer 14 maybe formed over the semiconductor substrate 12 using any known processand any suitable material known in the art. For example, thesemiconductor substrate 12 may be fabricated using silicon and the gateoxide layer 14 may include silicon dioxide (SiO₂) which has beenthermally grown or vapor deposited using well-known methods. The gateoxide layer 14 may be formed in various thicknesses to suit variousfabrication processes. By way of example and not limitation, the gateoxide layer 14 may have a thickness of about 50 angstroms or less. Foruse in state of the art 0.18 micron technology, a gate oxide layer 14having a thickness in the range of about 30 angstroms to about 40angstroms is currently preferred.

A polysilicon layer 16 is formed over the gate oxide layer 14 usingconventional deposition processes. The polysilicon layer 16 may also beformed in various thicknesses to suit various fabrication processes.However, it is currently preferred that the polysilicon layer 16 have athickness of about 400 angstroms to about 1000 angstroms, morepreferably about 600 angstroms. Dopants, such as boron, are subsequentlyimplanted into the polysilicon layer 16 followed by an anneal, as isknown to those of ordinary skill in the art.

After dopant implantation and anneal, nitrogen is implanted into theintermediate structure 10 at a dosage and energy sufficient to penetratethe polysilicon layer 16, the gate oxide layer 14 and at least a portionof the semiconductor substrate 12. The dosage and energy should besufficient such that the bulk of the nitrogen concentration is locatedentirely within the polysilicon layer 16 while a relatively smallconcentration of nitrogen penetrates below the polysilicon/gate oxideinterface 18. By way of example and not limitation, greater than about90% by weight of the total amount of nitrogen implanted may be locatedentirely within the polysilicon layer 16 while less than about 10% byweight of the total amount of nitrogen implanted may penetrate below thepolysilicon/gate oxide interface 18.

Typically, the nitrogen implantation takes place at room temperature ata dosage ranging from between about 1×10¹⁵ atoms/cm² to about 1×10¹⁶atoms/cm², and at an energy ranging from between about 10 keV to about40 keV. A desired nitrogen concentration profile of the intermediatestructure 10 after nitrogen implantation is illustrated in FIG. 2A,wherein areas of nitrogen concentration are indicated by line 21 andnitrogen concentration is increased in the direction of the arrow. Theconcentrations and the size of peak 23 depend on the thickness of thepolysilicon layer 16, the gate oxide layer 14 and the semiconductorsubstrate 12, as well as the implantation dosage and energy.Accordingly, FIG. 2A should be used for a qualitative, rather than aquantitative, understanding. Typically, not more than about 1×10¹⁴atoms/cm² to about 1×10¹⁵ atoms/cm² penetrates below thepolysilicon/gate oxide interface 18.

In later processing, an anneal may be performed at between about 850° C.and 1050° C. in an ambient nitrogen environment for between about 10seconds and 30 minutes to alter the nitrogen concentration profile ofthe intermediate structure 10. A desired nitrogen concentration profileof the intermediate structure 10 after anneal is illustrated in FIG. 2B,wherein areas of nitrogen concentration are indicated by line 21′ andnitrogen is increased in the direction of the arrow. As is evident fromFIG. 2B, the nitrogen concentration profile of the intermediatestructure 10 subsequent to anneal includes a relatively large nitrogenconcentration in the polysilicon layer 16 and gate oxide layer 14 at thepolysilicon/gate oxide interface 18 and a relatively small nitrogenconcentration within the gate oxide layer 14 and at the gateoxide/substrate interface 20. As with FIG. 2A, FIG. 2B should be usedfor a qualitative, rather than a quantitative, understanding.

In the intermediate structure 10 shown in FIG. 2B, the nitrogen at thepolysilicon/gate oxide interface 18 acts as a diffusion barrier toprevent diffusion of dopants from the polysilicon layer 16 into andthrough the gate oxide layer 14. Nitrogen concentrated at the gateoxide/substrate interface 20 also aids in preventing dopant diffusion.However, the concentration of nitrogen at the gate oxide/substrateinterface 20 is not so great as to substantially impair deviceperformance. Further, the nitrogen within the gate oxide layer 14 reactswith the pure oxide to form oxynitride, which has an increaseddielectric constant. Accordingly, the effective thickness of the gateoxide layer 14 is reduced. Utilizing the method of the presentinvention, an increase of about 5.0% to about 10.0% in the dielectricconstant of the gate oxide layer 14 may be achieved. Equivalently, areduction of about 5.0% to about 10.0% in the effective thickness of thegate oxide layer 14 may be achieved. Thus, an exemplary gate oxide layer14 having a thickness of about 30 angstroms may instead have a thicknessof about 28.5 angstroms if hardened using the method of the presentinvention.

As known to those of ordinary skill in the art, a metal gate strap 22(e.g., a metal silicide layer) is often disposed over the polysiliconlayer 16 to lower the resistance of the resultant semiconductor device(see, FIG. 3). However, semiconductor devices, for instance, surfaceP-channel transistors, having both P-type and N-type dopants mayencounter cross-diffusion when a metal silicide layer 22 is used. Asmore fully described below, using the method of the present invention,cross-diffusion may be substantially prevented.

Referring now to FIG. 3, a cross-sectional view of a second intermediatestructure 24 in the formation of a semiconductor device having a metalsilicide layer 22 formed over the polysilicon layer 16 thereof isillustrated. The metal silicide layer 22 may be formed of any suitablematerial known to those of ordinary skill in the art including, but notlimited to, tungsten silicide, and may be formed over the polysiliconlayer 16 using known processes. The metal silicide layer 22 may beformed over the polysilicon layer 16 subsequent to dopant implantationand anneal and may be formed in various thicknesses to suit variousfabrication processes. However, it is currently preferred that the metalsilicide layer 22 have a thickness of about 200 angstroms to about 1000angstroms, more preferably about 600 angstroms.

In this embodiment, nitrogen is implanted into the second intermediatestructure 24 at a dosage and energy sufficient such that the peak of thenitrogen concentration occurs within the metal silicide layer 22 and inthe polysilicon layer 16 at a silicide/polysilicon interface 26 while arelatively small amount of implanted nitrogen penetrates below thepolysilicon/gate oxide interface 18. Typically, the nitrogenimplantation takes place at room temperature at a dosage ranging frombetween about 1×10¹⁵ atoms/cm² and about 1×10¹⁶ atoms/cm² and at anenergy ranging from between about 10 keV and about 40 keV. A desirednitrogen concentration profile of the second intermediate structure 24after nitrogen implantation is illustrated in FIG. 4A, wherein areas ofnitrogen concentration are indicated by line 28 and nitrogenconcentration is increased in the direction of the arrow. As with theprevious embodiment that does not include a metal silicide layer 22, therelative nitrogen concentration and the size of peak 30 depend on thethickness of the layers 14, 16 and the semiconductor substrate 12, aswell as the implantation dosage and energy. Accordingly, FIG. 4A shouldbe used for a qualitative, rather than a quantitative, understanding.Again, as with the previously described embodiment, typically not morethan about 1×10¹⁴ atoms/cm² to about 1×10¹⁵ atoms/cm² penetrates belowthe polysilicon/gate oxide interface 18.

In later processing, an anneal may be performed at between about 850° C.and 1050° C. in an ambient nitrogen environment for between about 10seconds and 30 minutes to alter the nitrogen concentration profile ofthe second intermediate structure 24. A desired nitrogen concentrationprofile of the second intermediate structure 24 after anneal isillustrated in FIG. 4B, wherein areas of nitrogen concentration areindicated by line 28′ and nitrogen is increased in the direction of thearrow. As is evident from FIG. 4B, the nitrogen concentration profile ofthe second intermediate structure 24 subsequent to anneal includes arelatively large nitrogen concentration at the silicide/polysiliconinterface 26, a relatively smaller concentration of nitrogen at thepolysilicon/gate oxide interface 18 and a relatively smaller stillconcentration of nitrogen within the gate oxide layer 14 and at the gateoxide/substrate interface 20. As with FIG. 4A, FIG. 4B should be usedfor a qualitative, rather than a quantitative, understanding.

In the second intermediate structure 24 shown in FIG. 4B, the nitrogenconcentrated at the silicide/polysilicon interface 26 acts tosubstantially prevent cross-diffusion of dopants across the interfaceand the concentration of nitrogen at the polysilicon/gate oxideinterface 18 acts as a diffusion barrier to prevent diffusion of dopantsfrom the polysilicon layer 16 into and through the gate oxide layer 14.Nitrogen concentrated at the gate oxide/substrate interface 20 aids inpreventing dopant diffusion as well. However, as with the previousembodiment, the concentration of nitrogen at the gate oxide/substrateinterface 20 is not so great as to substantially impair deviceperformance. Further, the nitrogen within the gate oxide layer 14 reactswith the pure oxide to form oxynitride and, accordingly, increases thedielectric constant of the gate oxide layer 14 and reduces the effectivethickness thereof.

The following describes an exemplary method of forming a transistorhaving a gate oxide hardened according to the method of the presentinvention, which transistor is characterized by a relatively largenitrogen concentration at the silicide/polysilicon interface, arelatively smaller nitrogen concentration at the polysilicon/gate oxideinterface and a relatively smaller still nitrogen concentration withinthe gate oxide and at the gate oxide/substrate interface. This exampleis not in any way limiting of the scope of the invention.

EXAMPLE

A gate oxide layer having a thickness of about 30 angstroms wasthermally grown over a silicon substrate using methods known to those ofordinary skill in the art. A layer of polysilicon having a thickness ofabout 600 angstroms was subsequently deposited over the gate oxide layerfollowed by deposition of a layer of tungsten silicide having athickness of about 600 angstroms atop the polysilicon layer. Thedielectric constant of the gate oxide was determined and recorded.

Nitrogen was subsequently implanted into the gate stack, at roomtemperature, at a dosage of about 4×10¹⁵ atoms/cm² and at an energy ofabout 30 keV. The resultant nitrogen-implanted gate stack had a nitrogenconcentration profile approximating that illustrated in FIG. 4A with thebulk of the nitrogen concentration occurring within the metal silicidelayer and in the polysilicon layer at the silicide/polysilicon interfacewhile about 10% of the total amount of nitrogen implanted (i.e., about4×10¹⁴ atoms/cm²) penetrated below the polysilicon/gate oxide interface.At least a portion of the implanted nitrogen penetrated into the siliconsubstrate.

In later processing, an anneal was performed at about 1000° C. in anambient nitrogen environment for 20 seconds. The resultant gate stackexhibited a nitrogen concentration profile approximating thatillustrated in FIG. 4B with a relatively large concentration of nitrogenoccurring at the silicide/polysilicon interface, a relatively smallernitrogen concentration occurring at the polysilicon/gate oxide interfaceand a relatively smaller still nitrogen concentration occurring withinthe gate oxide and at the gate oxide/substrate interface.

The dielectric constant of the gate oxide layer was subsequentlydetermined and compared to that recorded prior to nitrogen implantation.It was found that the dielectric constant of the gate oxide layerincreased by about 7.0%, resulting in a reduction in the effectivethickness of the gate oxide layer of about 7.0%.

The present invention has been described in relation to particularembodiments that are intended in all respects to be illustrative ratherthan restrictive. It is to be understood that the invention defined bythe appended claims is not to be limited by particular details set forthin the above description and that alternative embodiments will becomeapparent to those of ordinary skill in the art to which the presentinvention pertains without departing from the spirit and scope thereof.

1. A method of forming a semiconductor structure, comprising: forming agate oxide over a semiconductor substrate, a polysilicon over the gateoxide, and a metal silicide over the polysilicon; and implantingnitrogen atoms through the metal silicide to penetrate the metalsilicide, the polysilicon, the gate oxide, and at least a portion of thesemiconductor substrate such that greater than about 90% by weight of atotal weight of the implanted nitrogen atoms are located proximate aninterface of the polysilicon and the metal silicide.
 2. The method ofclaim 1, wherein implanting nitrogen atoms through the metal silicide topenetrate the metal silicide, the polysilicon, the gate oxide, and atleast a portion of the semiconductor substrate comprises implantingnitrogen atoms such that less than about 10% by weight of the implantednitrogen atoms are located below an interface of the polysilicon and thegate oxide.
 3. The method of claim 1, wherein implanting nitrogen atomsthrough the metal silicide to penetrate the metal silicide, thepolysilicon, the gate oxide, and at least a portion of the semiconductorsubstrate comprises implanting nitrogen atoms such that from about1×10¹⁴ nitrogen atoms/cm² to about 1×10¹⁵ nitrogen atoms/cm² are locatedbelow an interface of the polysilicon and the gate oxide.
 4. The methodof claim 1, wherein forming a gate oxide over a semiconductor substrate,a polysilicon over the gate oxide, and a metal silicide over thepolysilicon comprises forming the gate oxide comprising an oxynitride.5. The method of claim 1, further comprising implanting dopants into thepolysilicon before implanting the nitrogen atoms through the metalsilicide.
 6. A method of forming a semiconductor structure, comprising:forming a semiconductor structure comprising a gate oxide overlying atleast a portion of a semiconductor substrate, polysilicon overlying thegate oxide, and a metal silicide overlying the polysilicon; implantingnitrogen atoms into the metal silicide, the polysilicon, the gate oxide,and at least a portion of the semiconductor substrate to form a nitrogenconcentration peak proximate an interface of the polysilicon and themetal silicide.
 7. The method of claim 6, wherein implanting nitrogenatoms into the metal silicide, the polysilicon, the gate oxide, and atleast a portion of the semiconductor substrate to form a nitrogenconcentration peak proximate an interface of the polysilicon and themetal silicide comprises forming the nitrogen concentration peakproximate the interface of the polysilicon and the metal silicidewithout forming a nitrogen concentration peak proximate an interface ofthe polysilicon and the gate oxide or proximate an interface of the gateoxide and the semiconductor substrate.
 8. The method of claim 6, whereinimplanting nitrogen atoms into the metal silicide, the polysilicon, thegate oxide, and at least a portion of the semiconductor substrate toform a nitrogen concentration peak proximate an interface of thepolysilicon and the metal silicide comprises annealing the semiconductorstructure to alter a concentration of nitrogen atoms located in each ofthe semiconductor substrate, the gate oxide, the metal silicide, and thepolysilicon.
 9. The method of claim 8, wherein annealing thesemiconductor structure comprises subjecting the semiconductor structureto a temperature of between about 850° C. and 1050° C. in ambientnitrogen.
 10. A method of forming a semiconductor structure, comprising:forming a semiconductor structure comprising a gate oxide, polysilicon,and a metal silicide, each overlying at least a portion of asemiconductor substrate; and implanting nitrogen atoms into the metalsilicide, the polysilicon, the gate oxide, and the at least a portion ofthe semiconductor substrate to form a nitrogen concentration profilecomprising a first nitrogen peak located adjacent an interface of themetal silicide and the polysilicon and a second nitrogen peak locatedadjacent an interface of the polysilicon and the gate oxide.
 11. Themethod of claim 10, wherein forming a semiconductor structure comprisinga gate oxide, polysilicon, and a metal silicide, each overlying at leasta portion of a semiconductor substrate comprises forming the gate oxideover the at least a portion of a semiconductor substrate, thepolysilicon over the gate oxide, and the metal silicide over thepolysilicon.
 12. The method of claim 10, wherein forming a semiconductorstructure comprising a gate oxide, polysilicon, and a metal silicide,each overlying at least a portion of a semiconductor substrate comprisesforming a metal silicide comprising tungsten over the polysilicon. 13.The method of claim 10, wherein implanting nitrogen atoms into the metalsilicide, the polysilicon, the gate oxide, and the at least a portion ofa semiconductor substrate to form a nitrogen concentration profilecomprises forming a third nitrogen concentration peak located adjacentan interface of the gate oxide and the semiconductor substrate.